Chlorinated aromatic compounds are generally used as solvents or raw materials in the manufacturing of medicinal and agricultural chemicals. For example, para-chlorotoluene is used in making pesticides and synthetic dyes, and para-dichlorobenzene in the making of synthetic dyes and moth control agents. More recently, para-dichlorobenzene is also used as raw material in making poly(phenylene sulfide), an important engineering plastics. Such new applications have resulted in the continuous increase in the demand for chlorinated aromatic compounds, especially the para-chlorinated products.
Conventionally, chlorinated aromatic compounds are made with liquid phase chlorination processes using homogeneous Lewis acids as catalysts. Examples of the Lewis acids for catalyzing the chlorination reaction between benzene rings and chlorine include SbCl.sub.5, FeCl.sub.3, and AIC13, etc. The conventional chlorination reaction utilizing Lewis acid as catalyst is often conducted at approximately one atmosphere pressure and 60.degree. C. in a batch reactor lined with corrosion-resistant material such as Teflon. The conventional chlorination process, however, has poor para selectivity and the chlorinated products often contain mixtures of ortho- and para-products. Sometimes, the products also contain polychlorobenzene as by-products. Typically, the para selectivity in the production of para-chlorotoluene is about 50-55%. The para selectivity in the production of dichlorobenzene from the convention process is slightly better at about 60-70%.
With the increasing concern over environmental pollution, the convention chlorination process suffers another shortcoming in that, after the chlorination reaction, it is often necessary to add large amounts of water to the reaction mixture to neutralize the Lewis acid catalyst in order to separate the organic layer (containing reaction products) from the aqueous layer (containing catalysts). This not only results in the dilution of the catalyst thus rendering it unrecyclable, it also creates waste water/waste acid disposal problems.
U.S. Pat. No. 4,327,036 (the '036 patent) discloses a liquid phase chlorination process using dichlorine monoxide as the chlorination agent. Acetic acid, trichloroacetic acid, or trifiuoroacetic acid is used as catalyst. The '036 patent does not address the issue of para selectivity. For example, in the chlorination of toluene, the product is primarily pentachlorotoluene. In the chlorination of chlorobenzene, the calculated para selectivity is only 60%.
A number of prior art disclosures have taught various heterogeneous catalytic chlorination processes which utilized various zeolites as catalysts for the chlorination of benzene rings. Zeolites are crystalline aluminosilicates of Group IA and Group IIA elements such as sodium, potassium, magnesium, and calcium. Chemically speaking, zeolites are represented by the empirical formula: M.sub.x/n.xAlO.sub.2.ySiO.sub.2.wH.sub.2 O, wherein M is a Group IA or Group IIA metal, n is the valence of the cation, x and y are an integer of 2 or greater, and w represents the water contained in the voids of the zeolite.
Japanese Pat. Pub. No. 77631 discloses a process using X-type zeolite as a catalyst in the gas phase chlorination reaction to make dichlorobenzene. The conversion of chlorobenzene was only 48.3%. Euo. Pat. Pub. No. 112,722 ('722 patent) discloses a process which used X-type zeolite as a catalyst in a liquid phase chlorination reaction of toluene. In a batch process, the convention of toluene was only 85%, and the selectivity of para-chlorotoluene was only 34.7%. Y-type zeolite has also been disclosed in the prior art as a catalyst in the liquid phase chlorination reaction to make dichlorobenzene. However, the selectivity of para-dichlorobenzene was only 71.8%. Both the X-type and Y-type zeolites are synthetic zeolites. The X-type zeolite is typically represented by a formula having a AlO.sub.2 /SiO.sub.2 ratio of about 86:106; whereas, the Y-type zeolite is typically represented by a formula having a relatively lower AIO2/SiO.sub.2 ratio of about 86:136.
In addition to the X-type and Y-type zeolites, L-type zeolite has also been disclosed in the prior art as a catalyst in the chlorination of benzene rings. The L-type zeolite is also a synthetic zeolite typically represented by the following formula: EQU K.sub.9 [(AlO.sub.2).sub.9 (SiO.sub.2).sub.27 ].22H.sub.2 O
As disclosed in Euo. Pat. Pub. No. 112,722, when an L-type zeolite was used in the chlorination of toluene, the conversion obtained was improved to 97.9%, and the ratio of parachlorotoluene/ortho-chlorotoulene was 0.5 (indicating a para selectivity of less than 33.3%). When an L-type zeolite was used in the chlorination of chlorobenzene, the conversion obtained was 85.6%, and the ratio of para-dichlorobenzene/ortho-dichlorobenzene was 0.124 (indicating a para selectivity of less than 11.0%).
Euo. Pat. Pub. No. 154,236 discloses a co-catalyst composition containing an aliphatic carboxylic acid and a zeolite catalyst to improve the conversion and para selectivity during the chlorination reaction. Specific examples include a co-catalyst composition containing L-type zeolite and acetic acid used in a batch chlorination process of toluene. The conversion of toluene was 99.8% and the selectivity of para-chlorotoluene was 70.2%. Another example involves L-type zeolite mixed with dichloroacetic acid in the chlorination of chlorobenzene. The conversion of chlorobenzene was 90.5% and the selectivity of para-dichlorobenzene was 92.7%.
Although the use of an aliphatic carboxylic acid as a co-catalyst with L-type zeolite can improve conversion and para selectivity of a chlorination reaction, it inevitably increases the raw material cost for making the chlorinated products. Furthermore, because of the need to separate aliphatic carboxylic acid from the final product after the completion of the reaction, substantial increase in equipment and production costs would incur.